The invention relates to the art of growing and pulling substantially monocrystalline bodies from melts of various crystalline inorganic materials. In particular, it relates to an improved crucible for containing the melt and an improved disposition of forming members and heating elements in combination with the improved crucible, for reducing the thermal gradients within the melt which are inherent in the prior art apparatus.
The invention is most useful when pulling a plurality of substantially monocrystalline objects (for example, twenty single crystal alpha-alumina tubular bodies) at one time. The prior art suggest that a large pot-shaped crucible be used with several die or forming members disposed in any arrangement in the crucible and adapted to extend above the surface of the melt in the crucible. (For example, see U.S. Pat. No. 3,687,633, FIG. 7). The crucible contents (including die or forming members) are then heated by means of an induction coil and susceptor arrangement or a resistance element surrounding the entire crucible. However, because of the large size of the crucible and because of the numerous die assemblies which lose heat at their upper surfaces rising above the melt level in the crucible, temperature gradients are built up throughout the melt which, in sequence, lead to undesirable convection currents which, together with the high temperatures, cause rapid attack on the crucible and forming members. The gradients and currents in the melt are higher in magnitude than those expected in smaller crucibles of lesser volume. Further, each addition of a forming member to the melt, and each increase in the size of the crucible may drastically affect the temperature distribution throughout the melt. A new problem in controlling the temperature is presented to the experimenter with each change in crucible size, change in the number of forming members of change in the arrangement of forming members.
The thermal gradients in prior art apparatus as described above are also increased by heating the crucible and contents therein in excess of the melting point of the contents. This higher heat imput is necessary in order to maintain the melt in the crystal growing region at its melting temperature. In particular, if the melt is to be shaped by a forming surface rising above the melt level, the film of melt on the forming surface and the surface itself must be conductively heated through the melt, requiring therefore, increased heat input to the crucible and contents.